The present invention relates most generally to the field of optical transmission of information. More particularly, the present invention relates to an integrated circuit apparatus and method for driving lasers to maintain desired optical of lasers while reducing power consumption.
In the field of telecommunications, lasers such as vertical cavity surface emitting lasers (VCSELs) and other opto-electronic devices are commonly used for the transmission of information along optical fibers and the like. VCSELs, in particular are especially desirable in today""s optical communication systems because they are efficient, small in size, readily assembled into arrays, and easy to manufacture.
Within optical communication systems utilizing VCSELs or other lasers, it is often desirable to control the parameters of the optical data signal being transmitted. For example, it is often desirable to control average power and amplitude of the signal. If the average power Pave is maintained properly, the laser may be modulated about the average power bias point at a modulation level necessary to achieve desired high and low light output power levels, Phigh and Plow.
An optical modulation amplitude (OMA) and an extinction ratio (ER) of the laser, defined as Phigh-Plow and Phigh/Plow, respectively, is commonly maintained within predetermined limiting values to maintain desired optical signal integrity. The limit values commonly are per specification such as the Synchronous Optical NETwork (SONET) or Gigabit Ethernet specification, or any other specification that the system is designed to meet.
Therefore, in order to obtain reliable and repeatable results in many fiber optic transmission applications, it is desirable to maintain both the average signal power and the OMA (or ER) within predetermined limit values. Unfortunately, laser characteristics change during the operation of the laser. In particular, as a laser such as a VCSEL is used to transmit optical data, the temperature of the operating laser and the environment which contains it, typically tends to increase, which may degrade laser performance. The OMA also changes as the temperature of the operating environment changes, and the change of the OMA with temperature is typically dependent on the particular laser used and the age of the particular laser. For example slope efficiency, a measure of optical output per current used to drive the laser, of the lasers may change due to temperature and age of the lasers. Automatic power control may be used to ameliorate these problems. Automatic power control is also used to account for laser threshold changes.
Similar to many other electronic systems, it is desirable to limit the power used by the laser drivers to drive the lasers. The limiting of the power used by the laser drivers to drive lasers result in reduction to power requirements and also reduces heat dissipation. Due to the reduced heat dissipation, the reduction in power requirements may also result in improvement of the laser performance due to reduced increase in temperature.
In one embodiment of the present invention, a power down feature is provided to disable one or more unused lasers so as to reduce power dissipation.
In another embodiment, a dual feedback system is used to provide feedback signals for the adjustment of modulation and bias currents delivered to a single laser or each laser in a laser array through the respective laser drivers. In this manner, the dual feedback system may be used to maintain average optical power and Optical Modulation Amplitude (OMA) within predetermined limit values. The feedback signals may be provided by one of the data lasers or by an additional control laser that operates at a lower frequency than the data lasers.
In yet another embodiment of the present invention, a transistor base leakage is used to emulate a large resistance element to result in a long time constant without the use of a large capacitor or large resistance.
In yet another embodiment of the present invention, a split power supply is provided to provide a lower supply voltage to a portion of a circuit, thus reducing power consumption of the overall circuit.
In yet another embodiment of the present invention, a low off-voltage is selected for a laser driver for driving longwave VCSELs.
In yet another embodiment of the present invention, feedback is used to reduce baseline wander and Inter Symbol Interference (ISI) that are produced by a low frequency dip appearing in the frequency response observed when using a PNP current source in conjunction with an NPN differential stage to drive a laser. A feedback resistor is employed in the PNP current mirror to substantially flatten the dip present in the frequency response.